1. Field of the Invention
This invention relates to a symmetrically wired device for measurement of the concentration of ions, especially hydrogen (H+) ions, in a measurement liquid.
2. Description of the Prior Art
The prior art discloses measurement devices for measurement of the ion concentration in a measurement liquid in different embodiments. Thus the pH of a measurement liquid can be recorded via asymmetrically wired measurement chains or via symmetrically wired measurement chains. The asymmetrically wired measurement chain comprises a reference electrode which is at the ground reference which is located in the measurement liquid and a pH-sensitive measurement electrode which is located in the measurement liquid, for example a glass electrode or a semiconductor electrode. A measurement signal referenced to the ground reference is recorded at the output of the measurement electrode. The measurement signal is dependent on the ion concentration in the measurement liquid.
A symmetrically wired device known from the prior art for measurement of the ion concentration is labeled in its totality with reference number 1 in FIG. 2. The device 1 comprises a pH-sensitive measurement electrode 3 which is located in the measurement liquid 2, a reference electrode 4 located in the measurement liquid 2, and a comparison electrode 5 located in the measurement liquid 2. The measurement liquid 2 is placed at a definable ground reference GND by the comparison electrode 5. The device furthermore comprises a subtractor 6 for forming a difference signal φpH which is referenced to the ground reference GND from the difference between the measurement signal φpH of the measurement electrode 3 and the reference signal φref of the reference electrode 4. The difference signal UpH is dependent on the ion concentration in the measurement liquid 2 or the pH of the measurement liquid 2. The measurement signal φpH and the reference signal φref are made as potentials. They are each routed via operational amplifiers 7, 8 wired as an impedance converter or isolation amplifier. The output signal of the subtractor 6 is likewise routed via an operational amplifier 9 which is wired as an impedance converter or isolation amplifier. The operational amplifiers 7, 8, 9 are used for common mode rejection, i.e. to reduce the fault susceptibility of the device 1. A conventional operational amplifier has common mode rejection of roughly 80 to 90 dB. This means that at an assumed allowable error at the output of an operational amplifier of 1 mV, at the input, a voltage fluctuation of up to 10 V can occur.
In the device known from the prior art for measurement of the pH of a measurement liquid, voltage fluctuations are caused for example by electrochemical noise potentials which arise between the comparison electrode and the measurement liquid, or by other disturbances which are coupled for example onto the measurement line. The magnitude of the noise potential is dependent on the material of the comparison electrode used and on the composition of the measurement liquid. The comparison electrode consists for example of high quality steel or of tantalum. The noise potential is added to the measurement signal and the reference signal equally. The difference formation provided in the symmetrical wiring eliminates the noise potential so that it does not act on the output signal of the device. The noise potential however leads to a rise in the individual potentials which are to be processed in the isolation amplifiers. The noise potential enters directly into the control range of the isolation amplifiers for the two potentials and in the following substructure. The electrochemical offset can be up to +/−1.4 V, in extreme cases even up to +/−2.8 V.
The increasing miniaturization of electronic components, especially to reduce the heat loss of active components, is generally accompanied by a reduction of the power supply voltage as well. Modern ASICs (Application Specific Integrated Circuits) for example are operated with power supply voltages which are Vcc=3 V or even less. In these circuits, the common mode range is limited to +/−3 V. When there is a noise potential or an electrochemical offset in the range of the power supply voltage, in the course of measurement of the pH the common mode range can be exceeded. The input circuit is then no longer able to linearly transmit the difference voltage and major measurement deviations can occur. In addition, the measurement signal can reach the trigger limit and can be simply cut off at a certain value, for example 3 V. For the case in which an analog subtraction circuit forms the difference of the two potentials, a following measurement instrument or a following A/D converter cannot detect any difference between a pH which is becoming smaller and a clipping of the potentials. A measurement is no longer possible.